1. Field of the Invention
This invention relates to ferrite-type hot-rolled stainless steel sheets that offer good workability and, in particular, excellent surface roughening resistance and high-temperature fatigue characteristics after working.
2. Description of the Related Art
Though somewhat less workable and less resistant to corrosion than an austenite type stainless steel, a ferrite type stainless steel is excellent in stress corrosion cracking resistance and also is inexpensive and hence has been widely applied to various kitchen fixtures and automotive exhaust components (exhaust manifolds, exhaust pipes, converter housings, mufflers and the like).
To improve the workability of such a ferrite type stainless steel sheet so as to be suitable for the above stated applications, it is common to fix impurity elements such as C and N in solid solution in the stock by the addition of elements such as Ti and Nb to a stainless steel stock. That technique is disclosed for instance in Japanese Patent Laid-Open Nos. 51-14811, 51-14812 and 52-31919. On the other hand, Japanese Patent Laid-Open No. 60-46352 discloses a highly corrosion-resistant ferrite-type stainless steel having a V content of 0.05 to 2.0% and a Cu content of 0.5 to 2.0%. This stainless steel thus has relatively high amounts of Cu so as to improve corrosion resistance. This stainless steel is exclusively useful as a cold-rolled steel material for automotive exterior trims, hot-water supply installations and other kitchen fixtures, and therefore is unconcerned with various mechanical characteristics required for a hot-rolled stainless steel, particularly high-temperature properties such as high-temperature fatigue resistance and the like.
In general, ferrite type stainless steel sheets are produced by heating a continuous casting slab and then subjecting the same to a series of process steps, i.e., hot rolling of the heated slab to obtain a hot-rolled sheet, annealing and pickling of the hot-rolled sheet, cold rolling of the annealed sheet, and final annealing and pickling of the cold-rolled sheet. If it were possible to omit any of the process steps, especially cold rolling and its subsequent steps, a conspicuous reduction of the plant investments and operating costs would be attained with respect to those omitted steps. This would mean that a ferrite type stainless steel sheet already of lower cost than an austenite type equivalent could be manufactured with further cost savings and shortened production time, and hence with great commercial merit.
Hot-rolled ferritic stainless steel sheets, however, are generally coarse in crystal grain after hot rolling and subsequent annealing as compared to cold-rolled ferritic stainless steel sheets, thus providing a steel product with a considerably roughened surface. Such crystal grain coarseness and surface roughness after working impair the aesthetic appearance of the steel product and moreover reduce the high-temperature fatigue properties of those steel components which are exposed to vibration as by engines at elevated temperatures, for example, automotive exhaust parts (exhaust pipes and the like). The last-mentioned phenomenon may be explained by the fact that, in a high-temperature fatigue environment, fatigue failure more readily occurs at grain boundaries than within crystal grains in a steel structure composed of large crystal grains, or such failure results from stresses localized on the roughened surface of the steel sheet.
The crystal grain sizes, which are closely associated with the surface roughening and fatigue failure of a steel sheet after working, may be adjusted to some degree with the varying temperatures and times for annealing. However, when annealed at a lower temperature and for a shorter time in order to render the crystal grain sizes microcrystalline, the steel sheet fails to completely recrystallize and keeps hot-rolled band structure in the vicinity of a central portion in the direction perpendicular to the plate thickness. This problem is responsible for a decrease of Rankford's value (r value) taken as a measure of deep drawing and elongation (El) and hence causes insufficient working performance. Consequently, good workability and excellent resistance to surface roughening and to high-temperature fatigue are difficult to achieve in a well-balanced manner with a ferrite type hot-rolled stainless steel sheet, and this poses a serious bottleneck in using the steel sheet for automotive exhaust parts requiring for those characteristics.